The invention relates to a method of manufacturing helically wound filaments, in which a filament wire is helically wound on a mandrel, the helically wound wire is heated while being held on the mandrel so that winding stresses in the wire are eliminated, and the mandrel is drawn out of the filament. Such a method is known from Dutch Patent Specification No. 17928 (Patent Treuhand, Mar. 15th, 1926).
This Dutch Patent Specification states that the step of drawing the mandrel out of the filament is an alternative to removing the mandrel by dissolving it. The step of removing the mandrel by drawing it out of the filament is attractive because the formation of large quantities of solution of mandrel material is not involved.
In order to obtain light sources of high quality, it is necessary that the filament has a substantially elongate filament wire portion on each side of the wound filament. For several lamp types it is necessary that these elongate portions are in line with the filament and consequently extend at least substantially longitudinally with respect to the mandrel during the manufacture of the filament. Such filaments cannot be manufactured by means of the method known from the above Dutch Patent Specification.